(1) Field of the Invention
This invention relates to three terminal devices that are solid electrochemical analogs of conventional semiconductor junction transistors. With standard external circuitry, the devices can be used to perform electrical functions common to semiconductor transistors such as ac voltage, current and power amplification.
(2) Prior Art
Transistor action is known where a species electrically emitted or generated in one region of a medium controls current drawn or collected from a nearby region. The species involved need not be limited to electrons and holes in semiconductors as was shown by Letaw and Bardeen (Journal of Applied Physics, Volume 25, No. 5, page 600, May, 1954) who constructed an electrochemical analog of a junction transistor using various oxidation-reduction couples in liquid electrolytes. Because of the use of liquid electrolytes, such devices can only be used over a limited range of operating conditions such as a normal atmospheric environment and temperatures.
Resistors, capacitors and transistors are examples of electrical components which have certain standard characteristics. A certain type of electrical input to the component is expected to produce a characteristic type of output. Many varieties of a given component may produce the same input-output transformation, such as, an air gap and an electrolytic capacitor. The need for using variations of a given component arises because a certain type of component may now allow sufficient range of electrical operation, sufficient durability, ability to withstand ambient conditions, etc. The variations of the given component can embody considerable differences in the chemical or physical principles of operation, in addition to differences in fabrication procedure and physical appearance. For example, two types of semiconducting devices are bipolar and field-effect transistors. In this light, the device discussed by Letaw and Bardeen is just one structure for electrochemically obtaining transistor action. Electrochemical conduction is discussed in more detail below.
In an ionic conductor (electrolyte), a charged form of an atom (an ion) is transported through the electrolyte toward an electrode under the action of an electric potential applied to the electrode by an external battery. Many electrolytes are liquids, like water, into which salts, like silver chloride are dissolved, producing ionic species, like Ag.sup.+ and Cl.sup.-. Referring to FIG. 1, a voltage applied between metal electrodes causes the ions to move in the liquid.
Many different ionic species can be transported in water. The ions can be used up, in the sense that A.sub.g will "plate out" on the negative electrode and Cl.sub.2 will be evolved at the positive electrode.
There are also solid electrolytes which usually conduct only a single ionic species. For example, doped ZrO.sub.2 conducts O.sup.=, the negatively ionized form of oxygen, as shown in FIG. 2. Porous platinum electrodes are attached to opposite ends of ceramic ZrO.sub.2. At least one of the electrodes must be surrounded by the neutral form of the conducting atom (oxygen in this case). For zirconium dioxide this is accomplished by placing the "cell" in an atmosphere containing oxygen, such as air. At the negative electrode, gaseous oxygen combines with electrons from the battery to form oxygen ions (O.sup.=) in the zirconium dioxide according to the following equation: EQU O.sub.2 (gas)+4e.sup.- (electrode).fwdarw.20.sup.= (electrolyte (1)
The O.sup.= ions are transported across the zirconium dioxide and expelled to the ambient atmosphere at the positive electrode as oxygen molecules. The electrons are given back to the electrode where they can flow back to the positive terminal of the battery and complete the current loop. The process is described by the following equation: EQU 20.sup.= (electrolyte).fwdarw.4e.sup.- (electrode)+O.sub.2 (gas) (2)
With zirconium dioxide, gaseous oxygen is transported from one side of the electrolyte to the other. This cell is sometimes called an oxygen pump, and could be used to generate a partial vacuum or overpressure if the regions around the electrodes were sealed off.
For a sodium beta alumina solid electrolyte, sodium atoms from a liquid or gaseous sodium reservoir would be transported as shown in FIG. 3. At the electrodes the following reactions would occur:
Positive electrode: EQU Na(reservoir).fwdarw.Na.sup.+ (electrolyte)+e.sup.- (electrode) (3) PA1 Negative electrode: EQU Na.sup.+ (electrolyte)+e.sup.- (electrode).fwdarw.Na(reservoir) (4)
This electrochemical behavior would eventually deplete the reservoir of Na surrounding the positive electrode. For continued operation, a means would be necessary to recirculate the Na. One possibility would be to completely immerse the cell in liquid or gaseous Na.
Discussing transistor action in more detail, a semiconductor transistor is a 3-terminal device having regions next to the terminals called the emitter, base and collector for bipolar (n-p-n, p-n-p) transistors. The corresponding regions for a field-effect transistor (FET) would be called source, gate, and drain, respectively.
There are three conventional modes of attaching external circuitry to the transistor. These are the common-base (CB), common-emitter (CE), and common collector (CC) modes. Each mode could be used to describe "transistor action". FIG. 4 illustrates the CB configuration.
A battery V.sub.E (usually called a "bias") is applied between the emitter and base terminals with a polarity (forward-bias) to force the electrically active species into the emitter and then into the base region as well. For a semiconducting transistor, the active species are electrons. An emitter current, I.sub.E, now flows out of the emitter terminal.
A battery V.sub.c is applied to the collector and base terminals through a load resistor R.sub.L. The polarity of the battery is such as to withdraw electrons from the collector (reverse bias) resulting in a collector current I.sub.c.
As a result, with the collector bias on, most of the electrons emitted into the base do not flow out of the base lead (terminal) to be registered as a base current I.sub.B (i.e., I.sub.B &lt;&lt;I.sub.E). Rather, most emitted electrons pass through the base and are collected by the collector so that I.sub.c .about.I.sub.E.
Further, the characteristics of the emitter-base-collector interfaces are such that increasing the collector bias does not increase the flow of either collector or emitter currents after a certain point. Rather, a collector current "saturates" at approximately the same value as the emitter current. These two features lead to a family of curves called the "collector characteristic" which is shown in FIG. 5, where V.sub.CB is the collector-base voltage and can be increased by increasing V.sub.c. Each curve represents the collector current that flows when the collector bias is increased for a series of fixed values of I.sub.E. Note that the collector current saturates at approximately the same value as the emitter current.
Concurrently, one would specify the "emitter characteristic" as in FIG. 6 where I.sub.E is plotted against V.sub.EB (=V.sub.E in this case) for various values of V.sub.CB. Whereas collector current saturation is the essential feature of the collector characteristic, the essential feature of the emitter characteristic is that at least for some range of V.sub.EB, the emitter-base impedance is low so that a small change in V.sub.EB gives a large change in I.sub.E. A device with collector and emitter characteristics which exhibit these essential features when connected in the CB configuration can be said to exhibit transistor action. If the device were wired in the CE or CC configurations, similar characteristic curves could be described which could be used to specify transistor action.
There are many uses for transistors, but a common one is a voltage amplifier. Adjusting V.sub.c and V.sub.E so that an operating point is established is shown by the dots on FIGS. 5 and 6. If an additional voltage source (the signal) produces a small change .DELTA.V.sub.EB in V.sub.EB about the emitter operating point, corresponding changes will occur in I.sub.E, I.sub.C and V.sub.CB. One can show that if R.sub.L is sufficiently large, then .DELTA.V.sub.CB &gt;&gt;.DELTA.V.sub.EB and the signal voltage has been amplified in the collector-base circuit. This can be achieved if the emitter and collector characteristics are like those of FIGS. 5 and 6. Current or current and voltage amplification can be achieved using the other modes of operation.
Referring to FIG. 7 which illustrates the Letaw and Bardeen device, a beaker contains water and several dissolved chemicals as well as metal electrodes. As an example, let the chemicals be such that there is a great abundance of Fe.sup.++ ions in solution. The metal electrodes will be called the emitter, base, and collector as shown. When the emitter-base electrodes are "forward biased" by V.sub.E, the Fe.sup.++ (ferrous) ions present in great abundance are converted to Fe.sup.+++ (ferric) ions near the emitter electrode according to the reaction Fe.sup.++ .fwdarw.Fe.sup.+++ (solution)+e.sup.- (electrode), and results in a current flow I.sub.E.
Normally the Fe.sup.+++ ions would start to diffuse away from the emitter region toward the base electrode where they would be reconverted back to Fe.sup.++ ions, according to Fe.sup.+++ (solution)+e.sup.- (electrode).fwdarw.Fe.sup.++ (solution) and resulting in a large base current I.sub.B. To defeat this and achieve "transistor action" however, the collector electrode is placed much closer to the emitter electrode than the base is. The collector is "reverse-biased" by V.sub.C and nearly all of the emitted Fe.sup.+++ ions are converted back to Fe.sup.++ ions at the collector before they can reach the base (so that I.sub.C .about.I.sub.E &gt;&gt;I.sub.B). This process will result in emitter and collector characteristics similar to those in FIGS. 5 and 6. The key to the device is placing the collector much closer to the emitter than the base. As the paper of Letaw and Bardeen shows, this concept can be made to work with a variety of dissolved chemical species in water.
Although the concept of Letaw and Bardeen provides a means of obtaining transistor action with an electrochemical device, it is limited to liquid electrolytes and the limited range of ambient temperatures and environments associated with such electrolytes. There is so far no known concept for fabricating electrochemical structures from solid electrolytes which will exhibit transistor action.